1. Field of the Invention
The present invention relates to optical polarizer materials and more specifically, it relates to double salts comprised from alkali metal carbonates which that have high birefringence and are useful as optical polarizers.
2. Description of Related Art
An electromagnetic wave (such as light from a laser) is characterized by its direction of propagation, frequency, amplitude and polarization. The polarization corresponds to the direction parallel to the plane (and normal to the propagation direction) in which the amplitude of the wave rises and falls. A polarizer acts to alter this direction, either by absorption or reflection of light waves with the incorrect polarization. Light from an incoherent source such as a light bulb consists of many super-imposed electro-magnetic waves with random, relative polarizations. Passing this light through a polarizer allows only those light waves with the desired polarization to pass through. An example of a polarizer of the first type (absorption) is Polaroid film found in many sunglasses. Polarizers of the second type (reflection) are formed using thin film dielectric coatings, parallel wire grids (used for wavelengths typically&gt;than 1 micrometer) and birefringent crystals (predominantly calcite (CaCO.sub.3)). Polarization altering components such as waveplates are almost exclusively made from quartz (SiO.sub.2). Lasers which are high peak power (large energy per pulse/pulse width) or high average power (large energy per pulse multiplied by the pulses per second) risk damaging the Polaroid or wire grid polarizers. Lasers such as these must use the thin film polarizers or calcite polarizers.
Calcite polarizers are the "best" polarizers for a number of applications. The extinction ratio of a polarizer is a measure of how well that polarizer operates at to produce a specific polarization state of light. By placing together two similar polarizers with their polarization directions at right angles to each other, theoretically no light should be transmitted through those two polarizers. The ratio of the intensity of the measured transmitted light to the intensity of the incident light is known as the extinction ratio. Polaroid polarizers typically have an extinction ratio of 1:10,000. Thin film polarizers have typical extinction ratios of 1:1000. Calcite polarizers have the highest extinction ratios in the range of 1:100,000 to 1:1,000,000. Thin film polarizers are typically manufactured for a single wavelength, and thus are wavelength sensitive. Calcite polarizers have a higher extinction ratio, have a damage resistance as high as that of thin film polarizers, and are broadband. They are usable in the range of 2000 nm to 250 unm.
Calcite is a naturally occurring mineral mined from the earth. The chemical formula is CaCO.sub.3. The best calcite is mined in northern Mexico, where it was deposited by naturally occurring geothermal processes. However, most of the calcite mined is unsuitable for optical use, due to veils, inclusions, and other crystalline defects. Also, although the theoretical transmission of calcite extends roughly from 200 nm to 4000 nm, the practical absorption in the far infrared and near ultraviolet (in the mined crystal) is limited by the ionic impurities (such as Fe) which were present in the water in which the calcite grew. These problems inherent in mined calcite would be reduced by synthetic growth methods. Unfortunately, if calcite is directly heated at atmospheric pressure, it decomposes to CaO and CO.sub.2 before it melts. (This is true of most carbonates.)
To grow calcite, it is necessary to duplicate the high pressure and temperature found in the earth. This entails growing calcite by a hydrothermal method. While hydrothermal methods are used extensively to grow quartz in industrial quantities, only experimental quantities of synthetically grown calcite are available. Industrial hydrothermal growth methods are much more expensive than low temperature melt growth methods, and calcite can not be grown by the standard low temperature melt growth methods. In addition, calcite grown using industrial hydrothermal methods contains microinclusions of water, degrading its performance in the infrared wavelength regime (due to optical absorption by the water inclusions) as well as in the ultraviolet spectral regime (due to scattering by the submicron water inclusions. Calcite remains, after more than a century, the principal material for Nicol prisms in the polarizing microscope. As natural stocks are depleted the need for growing synthetic CaCO.sub.3 in large 1-2" minimum size boules becomes more urgent. Thus, a need exists for a replacement material for calcite as a polarization material.